For more than a decade, my laboratory has been studying the role of costimulatory molecules and their receptors in the T-cell response, particularly in cancer immunity. The current proposal marks a shift of our emphasis from basic immunologyto clinical studies. Our main"goal is to address issues of general significance in translational research while identifying specific candidate molecules that can be useful for cancer therapy inhumans. Given our expertise in the area of T cell costimulation, we will be focusing on modulating T cell costimulatory molecules for cancer immunotherapy. Since the simplest method to elicit or inhibitthe function of receptors is to use anti-receptor antibodies, several groups, more recently including our own, have devoted considerable effort to testing the function of costimulatingantibodies in cancer therapy,in particular anti-CTLA4 and anti-4-lBB antibodies. While these studies have shown considerable promise, two major obstacles have delayed progress. First, the in vivo function of the antibodies cannot be easily recapitulated by in vitro assays, which makes it almost impossible to prescreen antibodies against human targets prior to clinical trials. Second, anti-CTLA4 antibodies have been shown to induce autoimmune diseases that seem to be concordant with cancer immunity. Here we propose to address these two issues systematically. .We will use our novel CTLA4-knock-inmouse model to evaluate the efficacy of .therapeutic anti-human CTLA4 antibodies, to dissect their mechanism of action, and to develop a novel strategy to tune down autoimmunityassociated with antibody therapy. Our proposed study will not only provide an example of how to identify therapeutic antibodies for human targets in mice, but also refute the notion that autoimmunityis a necessary price for tumor immunity. Moreover, our work also addresses fundamental issues on the specificity and function of regulatory T cells.